I. Field of the Invention
This invention relates generally to packaging containers and lidding stock; and in particular to heat sealed barrier multilayer laminates.
This invention relates generally to retort pouches and food packages. In one aspect it relates to a coextruded three layer film for use in retort pouches wherein each layer contains elastomer. In another aspect, the invention relates to a three layer laminate, one of which is a coextruded three ply film, for use in retortable pouches. In still another aspect the invention relates to a method of packaging food for retorting.
II. Description of the Prior Art
There is a trend in the food packaging industry to replace metal cans with plastic containers, notably pouches and trays containing sterilized or pre-cooked foods. Such retort pouches and trays offer many advantages over rigid metal packages: faster cook/sterilizing time, less shelf storage space, easier disposal, improved food taste, etc. The pouches are typically three layer laminates of an outer film (e.g., biax oriented nylon and/or polyester), a middle moisture and oxygen barrier film (e.g., aluminum foil, PVDC, or EVOH), and an inner heat seal layer film (PP, PPRC, or LLDPE or blends thereof). Adhesives to join the three layers together are typically of the polyurethane and/or maleated polyolefin types. (The resin abbreviations used herein are defined in "Description of Preferred Embodiments").
Large No. 10 cans (those containing about 3.2 liters of liquid food) have been slow to convert to flexible retort pouches and rigid or semirigid trays. Large plastic containers used to package foods have been limited to high acid foods (e.g., catsup) which do not require sterilization to kill botulism toxin. In such containers, LLDPE is often selected as the inner heat seal layer film because of its low cost, toughness and drop impact resistance. However, LLDPE cannot be used in large institutional-size retort pouches (volumetric content larger than 1 liter) which require sterilization, because it softens at typical high temperature sterilization or retort temperature (121.degree. C. and higher). Although PP or PRC films can be and are used in smaller size retort pouches (in the order of 0.25 to 1 liter content) because of their higher temperature resistance, they are unacceptable in the larger size pouches because they do not possess adequate toughness or drop impact strength to prevent breakage. With small containers, hydraulic forces resulting from vertical impact are not large. However, with large containers, the hydraulic forces become excessive, particularly on lateral seals. Because of this deficiency there exists a definite market need for institutional size pouches (greater than 1 liter and preferably 2 liter and larger) possessing a tough inner ply heat seal film which is resistant to high temperature retorting temperatures.
Blending elastomer into PP and PPRC is one well established method for improving film toughness and is being practiced commercially. PPRC monolayer film modified with PIB is used as the inner seal layer of medical enteral pouches used to deliver nutritional fluid to hospital patients unable to take food orally. These small 3-layer laminated pouches are typically one liter in size and weight approximately 2.2 lbs. AT least one side of these pouches consists of an all-plastic construction so that the fluid level can be monitored. In order to assure adequate impact toughness, these pouches are subjected to vertical drop tests. Typical vertical drop heights for these products are 4-6 feet. For greater drop height performance, smaller pouches (in the order of 0.5 liters) are sometimes used. However, pouches of this type are far too small for use as institutional food pouches.
European Patent Application 0,165,791 discloses a composite film structure comprising rubber-containing core and polyolefin resin. The skin layers of this composite however, contain no rubber. As demonstrated below, rubber (elastomer) in both the skin and core layers improves performance of the pouch.
There is a need in the food packaging industry for sealed rigid or semirigid polyolefin containers especially a retortable, easy open container with a barrier layer and heat seal lidding stock to make the container shelf stable.